The outermost regions of Saturn's hydrogen-helium atmosphere support ammonia, ammonium hydrosulfide, and water clouds. Saturn's storm features are not as pronounced or as long-lived as those observed on Jupiter, and Saturn has no weather feature as long-lived as Jupiter's Great Red Spot. However, isolated spots and cloud features are occasionally distinguishable from Earth. English astronomer William Herschel (1738–1822), for example, reported seeing small spots on Saturn's disk in 1780. Since that time, however, very few other features have been reported. The most dramatic recurring feature observed on Saturn is its Great White Spot. This feature was first observed by American astronomer Asaph Hall (1829–1907) on December 7, 1876, and six subsequent displays have been recorded. A Great White Spot was observed by the Hubble Space Telescope in 1990; smaller spots are observed in most Saturnian summer seasons.
All the white spots observed on Saturn are thought to be giant storm systems. When they first appear, the Great White Spots—the largest of these storms—are circular in form and some 12,420 mi (20,000 km) in diameter. Atmospheric winds gradually stretch and distort the spots into wispy bands, which can often be seen for several months. All of the Great White Spots have been observed in Saturn's northern hemisphere, with a recurrence interval equal to one Saturnian year (29.51 years). That the storms tend to repeat every Saturnian year suggests that they are a seasonal effect, with the storms being produced whenever Saturn's northern hemisphere is at maximum tilt toward the Sun. It is likely that storms also occur in Saturn's southern hemisphere when it is tilted toward the Sun, but the angle for viewing such events from the Earth is not favorable. It is assumed that the Great White Spots are produced by an up-welling of warm gas. Indeed, they have been likened to atmospheric "belches." In this manner the spots are similar to the cumulonimbus thunderheads observed in terrestrial storm systems. The prominent white color of the Saturnian storms is due to the freezing-out of ammonia ice crystals. These crystals form as the warm gas pushes outward into the frigid outer layers of the planet's atmosphere.